A number of devices for transmitting power and reducing motor output speed are currently known. Two such devices are the harmonic drive and the cycloidal drive.
The more well-known of these devices is no doubt the harmonic drive, having been introduced in the 1950's and currently being in wide-spread use. Harmonic drives offer a number of desirable operating characteristics, such as a high speed reduction ratio and high positional and rotational accuracy. However, harmonic drives also have several undesirable properties. First, harmonic drives require the use of flexible materials—particularly the material used to construct the flexspine. More particularly, harmonic drives use an elliptical wave generator that is forced into a toothed flexspine that rotates within a correspondingly toothed circular spine. The flexspine must be flexible so as to be deliberately deformed by and take on the shape of the elliptical wave generator. A special steel or another specialized deformable flexspine material is typically used for this reason. Additionally, the flexspine is subjected to continual elastic deformation during rotation of the wave generator. This deformation of the flexspine fatigues the material from which the flexspine is constructed, which fatigue limits the speed reduction and torque transfer that may be achieved with a harmonic drive.
Typical versions of the aforementioned cycloidal drive are of relatively complex construction and operation. Generally speaking, a cycloidal disc is provided for rotation within a chassis. A plurality of ring pins are associated with the chassis and distributed substantially equidistantly along a circular path for engagement by the cycloidal disc. An output shaft is also provided, and includes a number of output rollers that extend toward the cycloidal disc for engagement by receiving holes located therein. The cycloidal disc is eccentrically rotated by an input shaft mounted to an eccentric bearing. Rotation of the cycloidal disc causes the receiving holes therein to engage the output rollers of the output shaft. Because the output rollers are caused to travel about the periphery of the receiving holes in the cycloidal disc while the output shaft is rotated by the cycloidal disc, the rotational speed of the output shaft is reduced in comparison to the rotational speed of the cycloidal disc.
In addition to being of complex design, cycloidal drives are also typically not backdrivable. The eccentric rotation of the cycloidal disc also produces a vibration that may be transmitted through the input and output shafts if not compensated for. Contact between the output rollers and the receiving holes in the cycloidal disc may also lead to the wear of both components.
It can be understood from the foregoing commentary that there is a need for a power transmission/speed reduction device that does not suffer from the described drawbacks of known harmonic and cycloidal drives. Preferably, such a device will also be of more simplistic construction and of long life. Exemplary circular wave drive embodiments of the invention satisfy this need.